When producing polyurethane foams, a liquid or gaseous blowing agent is added to at least one of the reactive components (polyisocyanate and compounds with isocyanate-reactive hydrogen atoms, and in particular, polyols). This is then mixed with the other component and the obtained mixture is fed batchwise into a mold or continuously onto a conveyer belt, where the mixture expands and cures.
A number of processes have been widely used in the industry to produce the foam. In one process, liquids which evaporate at low temperature such as low molecular weight chlorofluorocarbons, methylene chloride, pentane, etc. which evaporate out of the liquid reactive mixture and form bubbles (physical foam production), are used. Furthermore, air may be forced into the reactive mixture or into one of the components (mechanical foam production). Finally, when expanding polyurethanes, water may be added to the polyol component as a blowing agent, which releases carbon dioxide as a foam-producing gas admixture with the isocyanate component, due to reaction with the isocyanate (chemical foam production).
In order to protect the environment and for occupational safety reasons, and also due to the comparatively high solubility of liquid carbon dioxide in the polyol component, liquid carbon dioxide has already been suggested several times as a blowing agent (GB-A 803,771, U.S. Pat. No. 3,184,419). However, these previous suggestions have not hitherto been used in an industrial scale process, obviously due to the difficulties in reducing the pressure of the reactive mixture from pressures of between 10 and 20 bar to atmospheric pressure in order to produce homogeneous foams. There is the problem, on the one hand, that carbon dioxide evaporates fairly suddenly after the pressure is reduced so that a very large increase in volume, by a factor of about 10, for example, occurs in the reaction mixture which is difficult to control and, on the other hand, the reactive mixture tends to delay the release of carbon dioxide which may be at 3 to 6 bar below the equilibrium vapor pressure of CO.sub.2 at the particular temperature, so that sudden and explosive evolution of the carbon dioxide takes place, resulting in large bubbles or holes being produced in the expanded material.
Furthermore, in order to produce a uniform expanded foam structure, it is known that finely dispersed air or nitrogen bubbles can be introduced into the liquid reactive mixture. These bubbles act as bubble seeds such that the local supersaturation of the physically dissolved or chemically produced blowing agent is inhibited.
However, when using physically dissolved carbon dioxide as a blowing agent, there is the problem that evolution of carbon dioxide takes place within the time interval during which the polyaddition reaction has barely initiated such that the froth obtained after evolution of the carbon dioxide is still very sensitive to the effects of shear forces. Shear forces acting on the froth can destroy the foam bubbles such that larger bubbles are formed and an irregular foam structure is produced. This is the case, in particular, when the foam bubbles reach a diameter at which the shape of the bubbles differs from a spherical shape, i.e. when the bubble volume in the froth occupies a fraction of the space which is greater than that corresponding to the closest packed arrangement of spheres. A bubble volume which corresponds to the closest packed arrangement of spheres is achieved when 0.5 parts by wt. of carbon dioxide at atmospheric pressure are released per 100 parts of reactive mixture.
During continuous production of blocks of expanded material (see Becker/Braun, Kunststoffhandbuch, vol. 7: Polyurethane, 1993, FIGS. 4.8 and 4.9, page 148) application of the froth onto the conveyer belt and its distribution over the width of the conveyer belt are regarded as problematical for the froth.
To overcome froth deposition and distribution problems on the conveyer belt, EP-A 645,226 suggests first distributing the polyurethane reactive mixture containing carbon dioxide under pressure over the width of the conveyer belt on the pressure side, in a so-called pressure compensation chamber, then decreasing the pressure in a pressure reduction zone extending over the width of the conveyer belt. The pressure reduction zone is designed in the form of gaps or a number of drilled holes extending over the width of the conveyer belt and representing an adequate resistance to flow. A foaming chamber is then provided, which extends transversely over the conveyer belt and widens in the direction of flow, from which the froth is intended to emerge with a flow-rate adapted to the speed of the conveyer belt. The disadvantages of EP-A 645,226 are the relatively long residence time of the froth in the foaming chamber, the relatively large proportion of foaming chamber wall and foaming chamber cross-section and the difficulty of avoiding differences between the flow-rate of the froth when discharged from the foaming chamber and that of the conveyer belt. In particular, enlarged foam bubbles, formed due to shear of the froth at the lower boundary wall of the foaming chamber and during transfer from the foaming chamber to the conveyer belt, can no longer disappear into the surroundings as a result of the bursting and discharge of included gas because they are covered by froth. Rather, these types of enlarged gas bubbles migrate to the lower face of the frothing material during the progressive polyaddition reaction and are included in the foam which is produced.